Yahoo also has one. Problem I find is you can't specify dates, airlines or routing. I tried this for a while and found that by the time I got the email (to be fair, read the email), the fare was either gone, or useless. A hypothetical example would be "ewr-las". I'd get an email for, say, $111 r/t. Great fare! Then, I find it's only on a Tuesday, return on Thursday, 3 stops on an airline that I don't care to accumulate miles on.
I've found that subscribing to various airlines vacation special emails gives a good indication of current *reasonable* competitive pricing.